Photoresists are polymer films which change their solubility response to a developer solution after the film has been exposed to an irradiation source, such as ultraviolet light or a beam of electrons or ions. As a consequence of the exposure, a different solubility rate results between the exposed and unexposed (masked over) portions of the photoresist film that yields a surface relief pattern after the development of the film.
Those photoresists which become more soluble in the exposed regions are referred to as positive acting. However, because the alteration of the solubility of the photoresist is only a relative change and even the less soluble unexposed portions of the photoresist dissolve to some extent, any process which enhances the developing rate difference (i.e. contrast) between the exposed relatively soluble and the unexposed, relatively insoluble photoresist portion is advantageous.
Positive photoresists are typically comprised of an aqueous alkaline soluble resin, such as novolac resin or poly (p-hydroxystyrene), and a diazonaphthoquinone sulfonic acid ester sensitizer. The resin and sensitizer may be applied by a method such as spin coating from an organic solvent or solvent mixture onto a substrate, such as silicon wafers and chrome plated glass plates. Developers that have been used to process the positive photoresists are aqueous alkaline solutions such as sodium silicate, potassium hydroxide, sodium hydroxide, tetramethyl ammonium hydroxide and ammonium hydroxide. Various salts have been added to the developers in order to increase the sensitivity of the resist. These salts include sodium phosphate, sodium borate, sodium carbonate, and sodium silicate. The addition of the corresponding acid will generate the salt in the developer, so the sodium cation is not a specific requirement. However, no improvement in contrast or resolution capability is realized.
The majority of existing positive photoresist systems can have a maximum contrast value (gamma) of three (3) to five (5) depending upon the processing condition. The problems associated with gammas of this order are evident in the reported performance. Typically, gammas of this order are obtained by using a weak developer and prolonged developing time with an attendant loss of throughput.
Gammas of greater than 5 have been achieved through the addition of certain surfactants to the developer. Quaternary ammonium surfactants increase the contrast of tetramethyl ammonium hydroxide developers and fluorocarbon nonionic surfactants increase the contrast in alkaline hydroxide, i.e. NaOH and KOH developers. These surfactants provide the high contrast, but the developer life is limited for repeated dip developing processes. Typically, the surfactant effect is greatly diminished after the first material has been developed with a resulting change in the exposure required. These developers lend themselves well to spray developing processes in which the developer is continuously replenished or used once and discarded as in a puddle process. A positive photoresist aqueous base developer that gives high contrast, high sensitivity and a stable bath life is desirable. The gamma obtained should be greater than five (5); the sensitivity, better than 40 mJ/cm.sup.2 ; and the bath life, greater than 400 wafers developed per gallon of developer.
The high contrast provides line-width control and process latitude in photoresist imaging. The high sensitivity allows for high throughput of wafers on the exposure tool. The long bath life provides a large number of substrates to be processed before changing the developer bath which reduces the cost of using the developer in terms of time and economics.
The line-width control is important in cases where fine lines are to be defined in the resist that covers steps or topography on the coated substrate. The linewidth of the patterned resist geometries change in dimension as the line crosses the step. The higher the contrast of the resist, the less the effect on dimensional change crossing a step. The process latitude afforded by the high contrast is a result of the ability to over-develop the exposed resist without affecting the unexposed resist in the adjacent areas. As a result, extremely small geometries of less than one micrometer can be patterned and the resist processing is less susceptible to change in conditions, such as exposure.
The high sensitivity is important to the throughput of the patterning process. The shorter the exposure time required, the more substrates can be processed through a given exposure tool in a given time. This feature is particularly important for direct write operations such as electron beam and ion beam lithography where throughput is critically dependent on the sensitivity of the resist system. For photolithography, the high sensitivity will allow setting the optics of the exposure tool to provide the best image quality. Finer lines can be patterned with smaller apertures which reduces the exposure level without sacrificing the throughput of an exposure tool. This is critical in projection aligners that project the image through a lens system onto the substrate.
High contrast developers can change after the first batch is processed. The changes are observed as a change in sensitivity and the corresponding change in the linewidth of the patterns. These changes prove to be detrimental to the linewidth control.
Accordingly, a need exists for an improved developer which provides superior contrast and the effectiveness of which does not change significantly as subsequent batches of substrates are developed; to the extent that of the order of more than an equivalent of 400 silicon wafers 100mm in diameter can be developed in four (4) liters of developer.